The invention relates to a ring-type voltage oscillator, and more particularly to a ring-type oscillator with improved duty cycle.
In recent years, the rapid growth of cellular communications systems has motivated an increasing demand for high performance integrated radio frequency (RF) components, such as frequency synthesizers. Phase-locked loop (PLL) techniques have been widely used in frequency synthesis to meet the stringent requirements of wireless standards. Among the function blocks in a PLL, VCO (Voltage-controlled oscillator) is a crucial circuit for determining the characteristics of a PLL. A desirable VCO offers a broad frequency band, realizes stable oscillation, and has low noise characteristics.
Ring-type oscillator is one kind of voltage-controlled oscillators. It is realized by cascading several delay cells to form a closed loop. FIG. 1 is a block diagram of a conventional ring-type oscillator 10. Operation of delay cells 12a, 12b and 12c in the ring-type oscillator is similar to an inverter and each delay cell generates a complement output signal with a time delay added to an input signal. The amount of time delay dictates the frequency at which the ring-type oscillator operates. The greater the delay time, the lower the frequency. That is, the frequency of the ring-type oscillator is inversely proportional to the total delay time of the delay cell in the loop. Ring-type oscillators provide advantages such as wide operating-frequency range and no extra processing step in a standard CMOS process, compared to other kind of VCOs, such as a LC-tank VCO.
However, VCOs including ring-type oscillators usually show poorer phase-noise performance, such as duty cycle distortion which may result in a poor bit-error rate and an inferior lock-in range of a clock and data recovery circuit. The duty cycle fluctuation of a ring-type oscillator may stem from the mismatch of transistors or extrinsic parameters, for example, device processing temperature, voltage and/or frequency of the frequency synthesizer. To solve this problem, a duty cycle correction circuit is usually added in a frequency synthesizer. However, this increases the power consumption and circuit complexity of the frequency synthesizer. Therefore, it is desirable to have a ring-type oscillator with an improved duty cycle.